1. Technical Field
The present disclosure relates to a micro flow sensor.
2. Related Art
A micro flow sensor measures a flow rate of a fluid by detecting a thermal transfer of a fluid. There have been proposed various types of micro flow sensors having flow paths and detection units integrated therein. For example, such micro flow sensors have been disclosed in Yoshiaki Tanaka and three others, “micro flow sensor for microreactor system”, Yokogawa Technical Report, Yokogawa Electric Corporation, 2008, Vol. 52, No. 4 (2008), p. 39-42, and JP-A-2010-230388.
FIGS. 18A and 18B are diagrams illustrating the configuration of a flow sensor disclosed as a conventional example in JP-A-2010-230388. FIG. 18A is a perspective view of the flow sensor and FIG. 18B is a cross-sectional view thereof taken along line B-B in FIG. 18A.
The flow sensor a includes a flow sensor chip 2 and a flow path forming member 3. A lower surface 3a of the flow path forming member 3 is connected to an upper surface 2a of the flow sensor chip 2. A micro flow path 4 is formed between the flow path forming member 3 and the flow sensor chip 2. A fluid to be measured, such as a gas, flows through the flow path 4. The flow path forming member 3 is made of transparent glass chips. On both ends of the flow path 4, a fluid inlet 4a and a fluid outlet 4b are formed. The fluid inlet 4a and the fluid outlet 4b have openings on an upper surface 3b of the flow path forming member 3. A gas as a fluid to be measured is introduced into the flow path 4 via the fluid inlet 4a, flows through the flow path 4, and is led out of the fluid outlet 4b. 
The flow sensor chip 2 includes a silicon base plate 5. As illustrated in FIG. 18B, an insulating film (thin film) 6 made of silicon nitride or silicon dioxide is formed on an upper surface of the silicon base plate 5. A flow rate detection unit 7 is formed at a position corresponding to a center of the flow path 4 on an upper surface of the insulating film 6. The flow rate detection unit 7 is coated with an insulating film 8 made of silicon nitride or silicon dioxide.
A cavity-like recess 2c is formed below the flow rate detection unit 7 in the center of the upper surface 2a of the flow sensor chip 2. Thus, a site, on the insulating film 6 including the flow rate detection unit 7 formed thereon, that covers the recess 2c is formed into a diaphragm shape. Thus, the flow rate detection unit 7 and the silicon base plate 5 are thermally blocked.
The flow rate detection unit 7 includes a heater and temperature measuring elements which are arranged on the insulating film 6. The temperature measuring elements are arranged on an upstream side and a downstream side at the same distance from the heater. The heater and the temperature measuring elements include, for example, platinum thin films. The flow rate detection unit 7 is configured to serve as a thermal detection unit. Furthermore, the flow rate detection unit 7 may include an ambient temperature sensor (not shown) disposed on the silicon base plate 5. When the heater of the flow rate detection unit 7 is energized, the heater is heated by a control circuit so as to be higher by a certain temperature than a temperature of a gas measured by the ambient temperature sensor. In this manner, the heater heats a gas flowing through the flow path 4.
While a gas does not flow through the flow path 4, the upstream side and the downstream side of the heater have a uniform temperature distribution. The temperature measuring element on the upstream side and the temperature measuring element on the downstream side exhibit resistance values corresponding to temperatures in a substantially equal manner. On the other hand, while a gas flows through the flow path 4, the uniform temperature distribution between the upstream side of the heater and the downstream side of the heater is lost. For example, the upstream side has a lower temperature, and the downstream side has a higher temperature. Then, a Wheatstone bridge circuit including the temperature measuring element on the upstream side and the temperature measuring element on the downstream side detects a difference in resistance value, that is a difference in temperature, between the temperature measuring elements. The flow sensor measures a flow rate of a gas flowing through the flow path 4 based on the detected temperature difference.
This flow sensor adopts a measurement principle of a so-called three-wire thermal system. In the three-wire thermal system, a temperature distribution to be formed by heating a heater is measured based on a temperature difference between two temperature sensors arranged on an upstream side and a downstream side of a heater at substantially equal distances from the heater. This temperature distribution depends on a flow rate. Therefore, the flow rate is calculated from this temperature distribution.